1. Field of the Invention
The present invention relates to a light scanning apparatus that emits a light beam and scans a photosensitive member with the emitted light beam and an image forming apparatus including the light scanning apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus, such as a copier or a printer, forms an image by developing, with toner, an electrostatic latent image formed on a photosensitive member by exposing the photosensitive member with a light beam such as laser light. A light scanning apparatus is employed as an apparatus configured to emit the laser light for scanning and exposing the photosensitive member.
The light scanning apparatus converts a light beam from a light source such as a semiconductor laser into a substantially collimated light beam, and then deflects the collimated light beam with a rotary polygon mirror. The light beam deflected by the rotary polygon mirror scans the photosensitive member in a substantially straight line shape.
In recent years, the light scanning apparatus is configured to expose the photosensitive member with a plurality of laser light beams emitted from a plurality of light emitting points, respectively, in order to support an increased speed of forming an image and an increased resolution of the image. In particular, a vertical cavity surface emitting laser (VCSEL) has been proposed as a light source of the light scanning apparatus owing to the easiness in forming an array of a large number of light emitting points.
In the light scanning apparatus, an optical sensor detects a light intensity of the light beam emitted from the light source, and the light intensity of the light beam emitted from the light source is controlled based on the detected light intensity. Unlike an edge emitting laser, the VCSEL does not have a backside-illumination laser light emitted in a direction opposite to the light beam guided to the photosensitive member. Therefore, in order to control the light intensity of the light beam emitted from the VCSEL, a light scanning apparatus has been proposed, which splits a part of the light beam emitted toward the photosensitive member with a beam splitter, guides one split light beam to the photosensitive member by deflecting the light beam with the rotary polygon mirror, and guides the other split light beam to the optical sensor to detect the light intensity of the light beam (see Japanese Patent Application Laid-Open No. 2006-091157). A ratio of the light intensity of the light beam passing through the beam splitter and guided to the photosensitive member to the light intensity of the light beam guided to the optical sensor is unambiguously determined by a reflectivity of the beam splitter. Therefore, the light intensity of the light beam arriving at the photosensitive member can be controlled to a target light intensity by controlling the light intensity of the light beam emitted from the VCSEL in accordance with the detected light intensity from the optical sensor.
However, when forming an image, a motor that rotates the rotary polygon mirror acts as a heat source, which causes the following problem. That is, when the rotary polygon mirror is rotated, an air flow is generated outward in a radial direction of the rotary polygon mirror. The air flow becomes then a heated air flow due to heat generated by the motor, and this heated air flow is directly blown to the optical sensor and a lens which is disposed between the beam splitter and the optical sensor. This may cause a variation in optical characteristics (such as refractive index) of the lens or a variation of an installation position of the lens due to thermal deformation of the installation position, which may change an optical path of the light beam traveling from the beam splitter to the optical sensor. When an optical sensor having a light receiving area large enough to permit the change of the optical path of the light beam is used to solve this problem, a cost of the optical sensor is increased.